RetroBSD

RetroBSD was developed with a portability in mind. Most of the kernel is architecture-independent (sys/kernel, sys/include). There is a small number of places, where a saved stack frame is directly referenced (look for u.u_frame) - it needs to be guarded by #ifdefs.

PIC32-related part of the kernel is placed in sys/pic32 directory. I would recommend to create sys/stm32 directory for this port. Some drivers, like SD card, console, GPIO or ADC probably could be moved to arch-independent place, say sys/drivers. Unfortunately, we cannot reuse USB stack (Microchip's copyright is too restrictive).

You can use sys/pic32/baremetal/Makefile configuration as a starting point, for simplicity. The challenge is to build a kernel, which can start and print a message on console. Copy a basic set of arch-dependent files from sys/pic32 to sys/stm32, and modify them (cpu.h, io.h, limits.h, machparam.h, startup.S, clock.c, cons.c, devsw.c, exception.c, machdep.c, mem.c, signal.c, sysctl.c). Design a memory map and create a linker script. All we need from MPU is to protect a kernel data region from the user programs.

Next step is to design a user/kernel API: how to pass args to syscalls and get returned data. This will help to build a syscall library (src/libc/thumb2/sys). All other assembler files can be ported from NetBSD or other sources.

The only "issue" with stm32F4xx (I like much) is the ram is split into 128kB + 64kB.
Not sure we can execute from those 64kB ram (so called CCM). Also no DMA to the CCM. Good for various buffers, however.

There are newer 32Fxx parts btw, with 256kB+ ram (F7 is 256kB + 64kB + 16kB + 4kB of ram). None model with 512kB chunk of ram yet.